A Paracrine Mechanism Accelerating Expansion of Human Induced Pluripotent Stem Cell-Derived Hepatic Progenitor-Like Cells

Search for Novel Plasma Membrane Proteins as Potential Biomarkers in Human Mesenchymal Stem Cells Derived from Dental Pulp, Adipose Tissue, Bone Marrow, and Hair Follicle

One of the drawbacks preventing the use of mesenchymal stem cells (MSCs) in clinical practice is the heterogeneous nature of their cultures. MSC cultures are not homogeneously formed by the MSCs and may contain non-mesenchymal cell types. Therefore, prior to use in clinics or research, complete characterization of MSCs should be performed to demonstrate the existence or absence of proper stem cell markers, many of which are happened to be cell-surface proteins. Unfortunately, the success of MSC characterization studies is limited due to the low specificity of the currently available cell-surface markers. Therefore, in this study, we aimed to investigate the plasma membrane (PM) proteins of MSCs isolated from human dental pulp (DP), adipose tissue (AT), bone marrow (BM), and hair follicle (HF) with the hope of proposing novel putative specific MSC markers. Differential-velocity centrifugation was used to enrich PM proteins. The isolated proteins were then identified by nLC-MS/MS and subjected to bioinformatics analysis. Proteins that were unique to each MSC type (CD9, CD10, CD63 for DP-MSCs; CD26, CD81, CD201, CD364 for AT-MSCs; Cd49a, CD49d for HF-MSCs; CD49e, CD56, CD92, CD97, CD156b, CD156c, CD220, CD221, CD298, CD315 for BM-MSCs) and common to all four MSC types (CD13, CD29, CD44, CD51, CD59, CD73, CD90) were identified. Uncharacterized proteins that have transmembrane (TM) domains were also detected. Some of the proteins identified in this study were the putative cell-surface markers that might be used for characterization of MSCs.

Synergistic modulation of signaling pathways to expand and maintain the bipotency of human hepatoblasts

Background

The limited proliferative ability of hepatocytes is a major limitation to meet their demand for cell-based therapy, bio-artificial liver device, and drug tests. One strategy is to amplify cells at the hepatoblast (HB) stage. However, expansion of HBs with their bipotency preserved is challenging. Most HB expansion methods hardly maintain the bipotency and also lack functional confirmation.

Methods

On the basis of analyzing and manipulating related signaling pathways during HB (derived from human induced pluripotent stem cells, iPSCs) differentiation and proliferation, we established a specific chemically defined cocktails to synergistically regulate the related signaling pathways that optimize the balance of HB proliferation ability and stemness maintenance, to expand the HBs and investigate their capacity for injured liver repopulation in immune-deficient mice.

Results

We found that the proliferative ability progressively declines during HB differentiation process. Small molecule activation of Wnt or inhibition of TGF-β pathways promoted HB proliferation but diminished their bipotency, whereas activation of hedgehog (HH) signaling stimulated proliferation and sustained HB phenotypes. A cocktail synergistically regulating the BMP/WNT/TGF-β/HH pathways created a fine balance for expansion and maintenance of the bipotency of HBs. After purification, colony formation, and expansion for 20 passages, HBs retained their RNA profile integrity, normal karyotype, and ability to differentiate into mature hepatocytes and cholangiocytes. Moreover, upon transplantation into liver injured mice, the expanded HBs could engraft and differentiate into mature human hepatocytes and repopulate liver tissue with restoring hepatocyte mass.

Conclusion

Our data contribute to the understanding of some signaling pathways for human HB proliferation in vitro. Simultaneous BMP/HGF induction, activation of Wnt and HH, and inhibition of TGF-β pathways created a reliable method for long-term stable large-scale expansion of HBs to obtain mature hepatocytes that may have substantial clinical applications.

Graphical abstract

Myofibroblasts control the proliferation of fetal hepatoblasts and their differentiated cholangiocytes during the hepatoblast-to-cholangiocyte transition

Mesenchymal cells in the liver provide the microenvironment for hepatoblasts expansion and differentiation. We have previously demonstrated that myofibroblasts (MFs) promoted hepatoblasts differentiation into cholangiocytes, whereas its role in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes remains elusive. Here, we investigated the role of MFs in regulating the proliferation of hepatoblasts and their differentiated cholangiocytes using an indirect coculture system. When cocultured with hepatoblasts, MFs promoted hepatoblasts differentiation into cholangiocytes and inhibited the proliferation and stemness of hepatoblasts. However, when hepatoblasts already differentiated into cholangiocytes, MFs promoted the differentiated cholangiocytes proliferation. In addition, hepatoblast proliferation genes such as hepatocyte growth factor (HGF), insulin-like growth factor-1 and 2 (IGF-1 and 2), midkine 1 (Mdk1), and pleiotrophin (Ptn) expression in MFs were down-regulated compared with their levels in fibroblasts. Our findings uncover the role of MFs in controlling the proliferation of hepatoblasts and their differentiated cholangiocytes, potentially providing a novel therapeutic strategy for cholangiocyte regeneration.

A hypothesis on paradoxical privileged portal vein metastasis of hepatocellular carcinoma. Can organ evolution shed light on patterns of human pathology, and vice versa?

Unlike other carcinomas, hepatocellular carcinoma (HCC) metastasizes to distant organs relatively rarely. In contrast, it routinely metastasizes to liver vasculature/liver, affecting portal veins 3-10 times more often than hepatic veins. This portal metastatic predominance is traditionally rationalized within the model of a reverse portal flow, due to accompanying liver cirrhosis. However, this intuitive model is not coherent with facts: 1) reverse portal flow occurs in fewer than 10% of cirrhotic patients, while portal metastasis occurs in 30-100% of HCC cases, and 2) portal vein prevalence of HCC metastasis is also characteristic of HCC in non-cirrhotic livers. Therefore, we must assume that the route for HCC metastatic dissemination is the same as for other carcinomas: systemic dissemination via the draining vessel, i.e., via the hepatic vein. In this light, portal prevalence versus hepatic vein of HCC metastasis appears as a puzzling pattern, particularly in cases when portal HCC metastases have appeared as the sole manifestation of HCC. Considering that other GI carcinomas (colorectal, pancreatic, gastric and small bowel) invariably disseminate via portal vein, but very rarely form portal metastasis, portal prevalence of HCC metastasis appears as a paradox. However, nature does not contradict itself; it is rather our wrong assumptions that create paradoxes. The 'portal paradox' becomes a logical event within the hypothesis that the formation of the unique portal venous system preceded the appearance of liver in evolution of chordates. The analysis suggests that the appearance of the portal venous system, supplying hormones and growth factors of pancreatic family, which includes insulin, glucagon, somatostatin, and pancreatic polypeptide (HGFPF) to midgut diverticulum in the early evolution of chordates (in an Amphioxus-like ancestral animal), promoted differentiation of enterocytes into hepatocytes and their further evolution to the liver of vertebrates. These promotional-dependent interactions are conserved in the vertebrate lineage. I hypothesize that selective homing and proliferation of malignant hepatocytes (i.e., HCC cells) in the portal vein environment are due to a uniquely high concentration of HGFPF in portal blood. HGFPF are also necessary for liver function and renewal and are significantly extracted by hepatocytes from passing blood, creating a concentration gradient of HGFPF between the portal blood and hepatic vein outflow, making post-liver vasculature and remote organs less favorable spaces for HCC growth. It also suggested that the portal vein environment (i.e., HGFPF) promotes the differentiation of more aggressive HCC clones from already-seeded portal metastases, explaining the worse outcome of HCC with the portal metastatic pattern. The analysis also offers new hypothesis on the phylogenetic origin of the hepatic diverticulum of cephalochordates, with certain implications for the modeling of the chordate phylogeny.

Direct and indirect coculture of mouse hepatic progenitor cells with mouse embryonic fibroblasts for the generation of hepatocytes and cholangiocytes

The widespread use of hepatocytes and cholangiocytes for regenerative medicine and tissue engineering is restricted by the limited number of hepatocytes and cholangiocytes; a simple and effective method for the expansion and differentiation of the hepatic progenitor cells (HPCs) is required. Recent studies demonstrated that mouse embryonic fibroblasts (MEFs) play an important role in supporting the proliferation of the mouse hepatic progenitor cells (mHPCs). However, the effect of direct and indirect coculture of MEFs with mHPCs on the differentiation of mHPCs is poorly studied. Herein, we show that mHPCs rapidly proliferate and form colonies in direct or indirect contact coculture with MEFs in the serum-free medium. Importantly, after direct contact coculture of the mHPCs with MEFs for 6 days, mHPCs expressed the hepatic marker albumin (ALB) and did not express the cholangiocyte marker CK19, indicating their differentiation into hepatocytes. In contrast, after indirect contact coculture of the mHPCs with MEFs for 6 days, mHPCs expressed the cholangiocyte marker CK19 and did not express the hepatic marker ALB, indicating their differentiation into cholangiocytes. These results indicate that direct and indirect contact cocultures of the mHPCs with MEFs are useful for rapidly producing hepatocytes and cholangiocytes.

An in vitro model of polycystic liver disease using genome-edited human inducible pluripotent stem cells

In the developing liver, bile duct structure is formed through differentiation of hepatic progenitor cells (HPC) into cholangiocytes. A subtype of polycystic liver diseases characterized by uncontrolled expansion of bile ductal cells is caused by genetic abnormalities such as in that of protein kinase C substrate 80 K-H (PRKCSH). In this study, we aimed to mimic the disease process in vitro by genome editing of the PRKCSH locus in human inducible pluripotent stem (iPS) cells. A proportion of cultured human iPS cell-derived CD13⁺CD133⁺ HPC differentiated into CD13^- cells. During the subsequent gel embedding culture, CD13^- cells formed bile ductal marker-positive cystic structures with the polarity of epithelial cells. A deletion of PRKCSH gene increased expression of cholangiocytic transcription factors in CD13^- cells and the number of cholangiocytic cyst structure. These results suggest that PRKCSH deficiency promotes the differentiation of HPC-derived cholangiocytes, providing a good in vitro model to analyze the molecular mechanisms underlying polycystic diseases.

Privileged portal metastasis of hepatocellular carcinoma in light of the coevolution of a visceral portal system and liver in the chordate lineage: a search for therapeutic targets

Hepatocellular carcinoma (HCC) disseminates systemically, but metastases occur in distant organs only in minority of patients, whereas HCC routinely metastasizes to liver and its vessels. HCC cells disseminate via hepatic veins, but portal veins are affected by metastasis more frequently than are hepatic veins, and correlates with poor prognosis. In this review, I suggest that privileged HCC portal metastasis occurs because of high levels of pancreatic family hormones and growth factors (PHGFs) in the portal blood. The analysis suggests that the appearance of the portal system carrying PHGFs in the evolution of invertebrate chordate (Amphioxus) led to the evolution of the liver in vertebrate; given that the portal pattern of HCC metastasis and selection of more-aggressive clones are PHGF dependent, PHGFs and their ligands constitute therapeutic targets.

Paracrine signals regulate human liver organoid maturation from induced pluripotent stem cells

A self-organizing organoid model provides a new approach to study the mechanism of human liver organogenesis. Previous animal models documented that simultaneous paracrine signaling and cell-to-cell surface contact regulate hepatocyte differentiation. To dissect the relative contributions of the paracrine effects, we first established a liver organoid using human induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs) and human umbilical vein endothelial cells (HUVECs) as previously reported. Time-lapse imaging showed that hepatic-specified endoderm iPSCs (HE-iPSCs) self-assembled into three-dimensional organoids, resulting in hepatic gene induction. Progressive differentiation was demonstrated by hepatic protein production after in vivo organoid transplantation. To assess the paracrine contributions, we employed a Transwell system in which HE-iPSCs were separately co-cultured with MSCs and/or HUVECs. Although the three-dimensional structure did not form, their soluble factors induced a hepatocyte-like phenotype in HE-iPSCs, resulting in the expression of bile salt export pump. In conclusion, the mesoderm-derived paracrine signals promote hepatocyte maturation in liver organoids, but organoid self-organization requires cell-to-cell surface contact. Our in vitro model demonstrates a novel approach to identify developmental paracrine signals regulating the differentiation of human hepatocytes.